Particularly in industrial settings, valves are used to carry and control a variety of gases, liquids and slurries over a wide range of temperatures and pressures. Many types of valves have emerged to meet the broad range of industrial applications, such as plug valves, ball valves, butterfly valves, gate valves, check valves, etc. Many valves are hand operated, while other valves have automated actuators to operate a valve from a remote location or to operate valves that are too large for human users to operate.
Most valves have a valving member (e.g. the plug in a plug valve, the ball in a ball valve, the disc in a butterfly valve, etc.), which is interposed in a flowpath. The valving member has an open position, which allows media to flow through the valve, and a closed position, which prevents media from flowing through the valve. Typically, a stem is used to move the valving member between its open and closed positions. The stem is usually connected to the valving member and extends out of the valve where it can be actuated, either manually or automatically.
Valves use one or more seals to control the media passing through the valve and prevent leakage. Leakage can be categorized as internal or external. Internal leakage refers to fluid flow around a seal and back into the flowpath. An example of internal leakage is a valve in the closed position that nevertheless permits some fluid flow through the valve. On the other hand, external leakage refers to leakage from inside the valve to the external environment. Because the very nature of valves is to control fluid flow, either type of leakage is naturally undesirable.
Stem seals are used to prevent external leakage around the stem. With today's increasing emphasis on emissions, the importance of effective stem seals are compounded. However, stem seals have many design parameters which complicate stem seal designs. For instance, in operation stems move (e.g. rotate), so a stem seal must be able to maintain its seal integrity during operation. Beyond normal operational movements, non-operational movements resulting from external forces (e.g. side loads, axial loads, etc.) are often experienced by stems. A stem seal design will preferably maintain its seal integrity when such non-operational movements are experienced. Thermal cycling is another design parameter. Since many valves undergo wide temperature swings during use, an ideal stem seal will preserve its sealing integrity through the dimensional changes that result in thermal cycling. Additionally, a stem will preferably maintain an external seal in high temperature environments, such as those experienced in fire conditions, which would consume many seal materials. Therefore, there is a need for a stem seal which can satisfy the foregoing design parameters.